The invention relates to an improved synthesis of alkyl and aryl esters of anhydroglycitol derivatives. These compounds are commercially interesting derivatives of the raw material sorbitol and other glycitols. The potential applications of these compounds are highly diverse. Esters of monoanhydrosorbitol (sorbitan) are widely used as emulsifiers (Span, Tween)1, 2. In addition, esters of dianhydrosorbitol (isosorbide) have many potential applications: as preservatives3-5, lubricants6, polymer stabiliser7, emulsifier in cosmetics8 9, dispersing agents for pigments10 or as plasticisers for vinyl resins11-15.
The dehydration of sorbitol, as an example of that of the glycitols, is shown in the diagram below: 
The current synthesis methods are usually based on acid-catalysed direct esterifications, sulphuric acid or p-toluenesulphonic acid being used as catalyst13,14. Base-catalysed reactions are also known; however, the reactions concerned here are usually transesterification reactions at high temperature (above 200xc2x0 C.)16-18. Furthermore, the use of acid ion exchange resins of the gel type as catalyst is also reported19,20; in this context yields of 61 and 63% for isosorbide dibutyrate and isosorbide dipropionate, respectively, are reported, starting from isosorbide.
In the case of the direct esterification the reaction equilibrium is shifted by removal of the water of reaction. This can be achieved by azeotropic distillation with toluene or xylene13,14,20, or by the use of a vacuum21. Yields in excess of 70% diester, starting from isosorbide, are not achieved with any of the above-mentioned methods.
The esterification of isosorbide is shown in the following equation: 
The invention relates to the synthesis of esters of dianhydrosorbitol and other dianhydroglycitols with high conversion (98-100%) and a substantially improved colour, as a result of which distillation of the product can be dispensed with. According to the invention use is made of a macroporous acid ion exchange resin as catalyst. In addition, an inert gas, such as nitrogen gas, is preferably dispersed through the reaction mixture in order to accelerate the removal of the water of reaction. A further improvement is obtained by increasing the turbulence of the reaction mixture, so that the removal of the water of reaction is further promoted. A reduced pressure of, for example, 10-50 mbar is also advantageous. The colour of the reaction mixture is substantially improved because the reaction temperature can be kept below 150xc2x0 C. Furthermore, addition of activated charcoal to the reaction mixture leads to a further reduction in the colour.
In addition to dianhydrosorbitol (isosorbide) as starting material it has also proved possible to use anhydrosorbitol (sorbitan) and even sorbitol as starting material. If the reaction temperature in the initial stage of the reaction is kept low (120-125xc2x0 C.), selective dehydration takes place, followed by esterification after raising the reaction temperature to 140-150xc2x0 C. Giacometti et al. 22,23 merely reported the possibility of in situ formation of anhydrosorbitol derivatives during the esterification of sorbitol with p-toluenesulphonic acid, without specifying experimental details for this.
Although ion exchange resins have been used as catalyst in the reaction for the dehydration19,21,24 of sorbitol, the conversions were too low (39-57%) and the reaction times usually too long (2-24 hours). Feldmann et al. (DE 3 041 673) reported the dehydration of sorbitol with the aid of a macroporous ion exchange resin, the water of reaction being removed with the aid of a stream of nitrogen. Despite the high yield of isosorbide (93%), the reaction mixture was severely discoloured and the reaction time was long (5 h).
Matyschok et al.21 also reported the synthesis of isosorbide esters with the aid of an acid ion exchange resin of the gel type (Wofatit KPS), in which context it must be mentioned that the alkanoic acids used by them have a short chain and thus high intrinsic acidity (acetic acid, propionic acid, butyric acid). The reported yields are, however, too low to be of industrial relevance (60-70%).
The process according to the invention preferably relates to the synthesis of diesters in accordance with the following equation: 
Surprisingly it has been found that a substantially improved method of preparation for dianhydrosorbitol diesters has been developed by a combination of techniques known per se. In view of the increasing industrial relevance of dianhydrosorbitol diesters, this meets an important need.
The process according to the invention can be used for the esterification of glycitols and the monoanhydro and dianhydro derivatives thereof. A glycitol is understood to be a sugar alcohol having at least 6 carbon atoms. These include, first of all, sorbitol, mannitol, iditol and other hexitols, but also higher analogues such as heptitols and glycitols derived from the di- and oligo-saccharides, such as lactitol, maltitol, and the like. The process according to the invention can also be used for glycitols (sugar alcohols) that cannot be converted to dianhydro analogues, such as pentitols (xylitol, etc.), in which case diesters and higher esters of the monoanhydro analogues (xylitan, etc.) are then formed.
The esterification can take place with any carboxylic acid, such as alkanoic acids, alkenoic acids, alkadienoic acids, cycloalkanecarboxylic acids and arenecarboxylic acids. The carboxylic acids can be either straight-chain or branched. Examples are propionic acid, hexanoic acid, octanoic acid, nonanoic acid, decanoic acid, lauric acid, stearic acid, cyclohexanecarboxylic acid, optionally substituted benzoic acids, phenylacetic acid, naphthalenecarboxylic acid, etc. The diesters of C3-C20 carboxylic acids are particularly advantageous. Mixtures of acids, in particular fatty acids of varying chain length, can also be used.
The esters of shorter chain carboxylic acids, such as C3-C6, can be used in the main as solvents, those of medium chain length alkanoic acids, in particular of C6-C12 carboxylic acids, are outstandingly suitable as plasticisers and the longer chain length, for example C12-C18, carboxylic acids are mainly usable as lubricants. If desired, monoesters of dianhydroglycitols can also be obtained by using smaller amounts of fatty acids, for example 1 to 2 mol per mol (anhydro)glycitol. What is concerned in this case is then mainly the preparation of emulsifiers, such as the monoesters of C12-C20 alkanoic acids or alkenoic acids and monoaryl and monoaralkyl esters.
The choice of the catalyst resin is important. This is an acid catalyst resin of the macroporous or macroreticular type. In contrast to resins of the gel type, these are resins with a relatively high degree of crosslinking and consequently a high porosity. A description of suitable resins is to be found in standard works on catalyst resins, such as xe2x80x9cIon Exchangersxe2x80x9d by Konrad Dxc3x6rfner, published by De Gruyter, Berlin, 1991, in particular pages 22-23 thereof. Examples of suitable resins are the commercially available resins, such as Amberlyst-15-wet, Amberlyst-15-dry, Amberlyst-16-wet and Amberlyst-36-dry from Rohm and Haas, and comparable resins from other suppliers.